The instant invention relates to the protection of metal surfaces against corrosion and more particularly, to a method of protecting metal surfaces, such as the exterior surface of a steel pipe, against corrosion at temperatures of up to 250.degree. F. and to a primer which is adapted for use in the method.
Steel pipes have been widely utilized for transporting fluids, such as water, oil, and gas, for many years in both above ground and underground applications. However, it has been found that steel pipes utilized in underground applications tend to rapidly corrode, due to both oxidation and electrolysis unless the exterior surfaces thereof are adequately protected with suitable corrosion resistant materials. Consequently, various protective tape coating systems have been developed over the years for protecting the exterior surfaces of underground steel pipes against corrosion. Many of the currently available tape coating systems comprise primers which are applied directly to the exterior surfaces of metal pipes and protective adhesive tapes which are spirally wrapped around the exterior surfaces of pipes over the respective primers thereon. The heretofore available primers have generally comprised elastomers, resins, colorants, fillers and solvents. However, it has been found that primers of this type are frequently responsible for tape disbondment when they are utilized on underground pipes carrying relatively high temperature materials. Specifically, it has been found that primers of this type tend to oxidize and become plasticized by oils migrating from tape adhesives when they are exposed to temperatures in excess of approximately 180.degree. F. It has been further found that tape disbondment resulting from primer failure is generally accelerated when cathodic protection is applied as a secondary type of corrosion protection to an underground pipe system. Hence, there is a recognized need for an improved primer for protecting the exterior surfaces of underground pipe systems against corrosion in applications where the pipe systems are exposed to temperatures of between approximately 180.degree. F. and 250.degree. F.
The instant invention provides an effective primer which can be utilized in combination with an adhesive tape for protecting the exterior surfaces of an underground metal pipe system against corrosion at temperatures of up to 250.degree. F., even in applications where cathodic protection is applied to the pipe system. More specifically, the instant invention provides an effective primer which is capable of adhering to the metal surfaces of a pipe at elevated temperatures in the presence of cathodic anticorrosion protection. Still more specifically, it has been found that by adding a heat reactive phenolic resin to a conventional corrosion resistant primer comprising a butyl elastomer, an aliphatic resin, fillers and colorants, and one or more solvents, the resulting primer formulation can be effectively utilized in a tape corrosion protection system for protecting metal surfaces at temperatures of up to 250.degree. F. in the presence of cathodic corrosion protection. In particular, the primer of the instant invention comprises between approximately 1% and 10% by weight of a butyl elastomer, between approximately 1% and 20% by weight of an aliphatic hydrocarbon resin, a quantity of a heat curable phenolic resin equal to between approximately 2% and 20% of the weight of the aliphatic hydrocarbon resin, and between approximately 26% and 98% of a solvent. The aliphatic hydrocarbon resin utilized in the primer is selected so that it has a softening point of between approximately 70.degree. F. and 150.degree. F., and it is selected so that it is compatible with the butyl elastomer. The heat curable phenolic resin is selected so that it is compatible with both the butyl elastomer and the aliphatic hydrocarbon resin, and so that it crosslinks at between approximately 180.degree. F. and 250.degree. F.. The solvent is selected so that it is compatible with the butyl elastomer, the aliphatic hydrocarbon resin, and the heat curable phenolic resin.
In the preferred formulation of the primer of the instant invention, the butyl elastomer has a Mooney viscosity of between approximately 40 and 50 at 100.degree. C., and a Mooney viscosity of between 45 and 55 at 125.degree. C.. Further, the solvent preferably comprises either an aromatic petroleum solvent or an aliphatic petroleum solvent, and the heat curable phenolic resin preferably comprises either bromo methyl alkylated phenol formaldehyde resin, or methyl alkyl substituted phenolic resin.
The method of protecting a metal surface of the instant invention comprises applying a primer of the above described type to the metal surface, allowing the primer to at least partially cure, and applying a protective adhesive tape to the metal surface over the primer. The metal surface preferably comprises an exterior surface of a metal pipe which has been cleaned, such as by sandblasting, prior to applying the primer thereto and the primer is preferably only allowed to partially cure before the adhesive tape is applied. The adhesive tape preferably comprises a conventional butyl based adhesive tape of the general type which is adapted for use in protecting the exterior surfaces of metal pipe.
It has been found that the method of the instant invention can be effectively utilized for applying the primer of the instant invention to steel pipes in order to protect the pipes against corrosion. In this connection, it has been found that the primer of the instant invention as applied by the above-described method can be effectively utilized for protecting metal pipes against corrosion at temperatures of up to 250.degree. F., and that the primer can even be effectively utilized for protecting metal pipes which are further protected against corrosion by cathodic protection. Hence, it has been found that that the primer and method of the instant invention represent effective solutions to the problem of protecting underground pipes carrying relatively high temperatures liquids against corrosion.
Accordingly, it is a primary object of the instant invention to provide an improved primer for protecting a metal surface against corrosion.
Another object of the instant invention is to provide an effective primer for protecting the exterior surfaces of a metal underground pipe utilized for carrying a relatively high temperature liquid against corrosion.
A further object of the instant invention is to provide an effective primer for protecting the exterior surfaces of a metal underground pipe utilized for carrying a relatively high temperature liquid against corrosion regardless of whether or not the pipe is subjected to cathodic protection.
An even still further object of the instant invention is to provide a method of protecting the exterior surfaces of a metal underground pipe utilized for carrying a relatively high temperature liquid against corrosion.
An even still further object of the instant invention is to provide an effective method of protecting the exterior surfaces of an underground metal pipe utilized for carrying a relatively high temperature liquid against corrosion, regardless of whether or not cathodic protection is applied to the pipe.
Other objects, features and advantages of the invention shall become apparent as the description thereof proceeds when considered in connection with the accompanying illustrative drawing.